1. Field of the Invention
The invention relates to a control device and a control method for a vehicle. More particularly, the invention relates to a control device and a control method that are applied to a vehicle capable of forming a specific gear shift stage even in a case where an electrical failure occurs in a solenoid valve.
2. Description of Related Art
In the related art, a technique for forming a specific gear shift stage according to a gear shift stage prior to a failure in which all solenoid valves are de-energized, without using electrical control, even in a case where the failure occurs in an automatic transmission forming the gear shift stage by using the solenoid valves to control an engagement device is known (for example, refer to Japanese Patent Application Publication No. 2005-265101 (JP 2005-265101 A)). In this type of automatic transmission according to the related art, a switching valve that is switched between a normal position and a failure position according to hydraulic pressure supplied by using a normally closed (NC) solenoid valve and hydraulic pressure supplied from a normally open (NO) solenoid valve is used, and a 3-speed stage is mostly formed when the failure occurs in 1- to 4-speed stages and a 5-speed stage is mostly formed when the failure occurs in a 5- or 6-speed stage.
However, the hydraulic pressure supplied by using the NC solenoid valve is low immediately after switch from an N range to a D range, and thus the switching valve may be switched to the failure position, even during a normal operation, by the hydraulic pressure supplied from the NO solenoid valve in the automatic transmission of the related art.
In, for example, Japanese Patent Application Publication No. 2011-190851 (JP 2011-190851 A), a technique for using two relay valves, to ensure opposing pressure against hydraulic pressure supplied from a NO solenoid valve and suppress switch of a switching valve to a failure position even in a case where hydraulic pressure supplied by using a NC solenoid valve during a normal operation is insufficient is disclosed.
Although the switching valve is switched to the failure position in a case where the failure in which all of the solenoid valves are de-energized (hereinafter, also referred to as all failure) occurs in the automatic transmission according to the related art, the switching valve is not always switched to the failure position even when some of the plurality of NC solenoid valves fail (hereinafter, also referred to as partial failure). For example, the gear shift stage prior to the failure is maintained in some cases, with the switching valve remaining at the normal position, depending on a relationship between the engagement device corresponding to the solenoid valve that fails and the gear shift stage prior to the failure and the size of the hydraulic pressure supplied from the NO solenoid valve.
Although a specific low-speed stage (3-speed stage) is formed when the all failure occurs during low-speed traveling (1- to 4-speed stages) and a specific high-speed stage (5-speed stage) is formed when the all failure occurs during high-speed traveling (5- or 6-speed stage) in the automatic transmission according to the related art, the specific low-speed stage is internally established in some cases even during the high-speed traveling due to, for example, a misoperation by a driver during the partial failure.
Accordingly, in the automatic transmission according to the related art, downshift to the specific low-speed stage may occur even during the high-speed traveling when the hydraulic pressure supplied from the NO solenoid valve increases in a case, for example, where the misoperation by the driver and the partial failure occur simultaneously. As a result, over-revolution of an internal combustion engine may be caused.
It is conceivable, as in Japanese Patent Application Publication No. 2011-190851 (JP 2011-190851 A), to use a plurality of the relay valves and the like to ensure the opposing pressure against the hydraulic pressure supplied from the NO solenoid valve and suppress the switch of the switching valve to the failure position when the partial failure occurs. However, when the number of the valves and the like constituting a hydraulic circuit increases, the hydraulic circuit may become more complex, oil passages connecting the valves to each other may become more complex, and control may become more complex.